Mass selective detection of ions in an ion trap is an important step of mass analysis in an ion trap mass spectrometry system. The motion of the ions trapped in an ion trap is approximately governed by the following equation:
                                          a            z                    =                                                    -                2                            ⁢                              a                r                                      =                                                            -                  16                                ⁢                zU                                            m                ⁢                                                                  ⁢                                                      ω                    rf                    2                                    ⁡                                      (                                                                  r                        0                        2                                            +                                              2                        ⁢                                                  z                          0                          2                                                                                      )                                                                                      ⁢                                  ⁢                              q            z                    =                                                    -                2                            ⁢                              q                r                                      =                                          8                ⁢                zV                                            m                ⁢                                                                  ⁢                                                      ω                    rf                    2                                    ⁡                                      (                                                                  r                        0                        2                                            +                                              2                        ⁢                                                  z                          0                          2                                                                                      )                                                                                                          Equation        ⁢                                  ⁢        1            where au and qu (u refers to ion motion in the r- and z-dimensions) are Mathieu parameters, U is the DC potential, V is the maximum RF amplitude, ωrf is the frequency of the RF, r0 is the internal radius of the ring electrode, and the z axis is perpendicular to the endcap electrodes. E.d. Hoffmann and V. Stroobant, Mass Spectrometry: Principles and Applications, Second Edition ed: John Wiley &Sons, LTD, 2002. The frequencies of the ion trajectory can be described by:
                              ω          =                                    (                              n                ±                                  β                  2                                            )                        ⁢                          ω              rf                                      ⁢                                  ⁢                              n            =            0                    ,          1          ,                      2            ⁢                                                  ⁢            …                                              Equation        ⁢                                  ⁢        2            and β is defined as:
                              β          u          2                =                              a            u                    +                                    q              u                                                                        (                                                            β                      u                                        +                    2                                    )                                2                            -                              a                u                            -                                                q                  u                  2                                                                                                                                                                          (                                                                                          β                                u                                                            +                              4                                                        )                                                    2                                                -                                                  a                          u                                                -                                                                                                                                                                          q                          u                          2                                                                                                                                    (                                                                                                β                                  u                                                                +                                6                                                            )                                                        2                                                    -                                                      a                            u                                                    -                          …                                                                                                                                                  +                                    q              u                                                                        (                                                            β                      u                                        -                    2                                    )                                2                            -                              a                u                            -                                                q                  u                  2                                                                                                                                                                          (                                                                                          β                                u                                                            -                              4                                                        )                                                    2                                                -                                                  a                          u                                                -                                                                                                                                                                          q                          u                          2                                                                                                                                    (                                                                                                β                                  u                                                                -                                6                                                            )                                                        2                                                    -                                                      a                            u                                                    -                          …                                                                                                                                                                            Equation        ⁢                                  ⁢        3            
The mass selective ion detection can be done in destructive or non-destructive ways. In the destructive way, the most common of which is mass selective instability scanning, RF amplitude is scanned up and ions are moved towards stability boundaries or resonance points, becoming instable. G. C. Stafford, P. E. Kelley, and D. R. Stephens, “Method of mass analyzing a sample by use of a quadrupole ion trap,” vol. 4540884. United States: Finnigan Corporation, 1985. Once unstable, the ions are ejected sequentially according to mass-to-charge ratios of the ions. Other scan methods have been used by back scanning of the RF amplitude to a resonance point (G. C. Stafford, P. E. Kelley, and D. R. Stephens, “Method of mass analyzing a sample by use of a quadrupole ion trap,” vol. 4540884. United States: Finnigan Corporation, 1985) or scanning RF frequency instead of RF amplitude. The ions ejected are detected using an ion detector such as an electron multiplier and microchannel plate. These ion detectors usually rely on particle multiplication, which is susceptible to an increase of pressure.
Non-destructive ion detection was also developed for ion trap mass analysis using image current detection. M. Soni, V. Frankevich, M. Nappi, R. E. Santini, J. W. Amy, and R. G. Cooks, “Broad-Band Fourier Transform Quadrupole Ion Trap Mass Spectrometry,” Anal. Chem., vol. 68, pp. 3314-3320, 1996; and D. E. Goeringer, R. I. Crutcher, and S. A. McLuckey, “Ion remeasurement in the radio frequency quadrupole ion trap,” Analytical Chemistry (Washington; VOL. 67; ISSUE: 22; PBD: 15 Nov. 1995, pp. pp. 4164-4169; PL:, 1995. Image current detection is a technique that is similar to that used in an ion cyclotron resonance (ICR) instrument, in which a transit of image current induced by ion motions is recorded, and frequency components are subsequently derived by Fourier transform. Wide band detection to acquire secular frequencies of ions suffers from the high pressure of ion trap operation, and noise by RF and other electric signals.
There is a need for systems and methods that overcome the above deficiencies.